Cystic fibrosis (CF) is a common genetic disorder characterized by defective epithelial chloride transport and progressive, lethal lung disease. Chronic sinusitis, which clinically plagues approximately 50% of CF patients, is treated at Stanford with a combination of maxillary antrotomy placement followed by monthly maxillary sinus tobramycin lavage. A total of five patients have been enrolled in protocol 438, an evaluation of maxillary sinus treatments in patients with antrostomies. This randomized, double-blind comparison of maxillary sinus tobramycin lavage versus placebo is designed to document the effectiveness of tobramycin lavage and determine the rate of sinusitis recurrence. Nasal mucosa transepithelial potential differences are similar to maxillary sinus transepithelial differences, except that maxillary sinus transepithelial potential differences, during baseline determinations, are more sensitive to the effects of local chronic infection.